Abstract
Introduction
Although total hip arthroplasty is a very successful operation, complications such as: dislocation, aseptic loosening, and periprosthetic fracture do occur. These aspects have been studied in large populations for traditional stem designs, but not for more recent short stems. The design rationale of short stems is to preserve bone stock, without compromising stability. However, due to their smaller bone contact area, high peak stresses and areas of stress shielding could appear in the proximal femur, especially in the presence of atypical bone geometries. In order to evaluate this aspect, we quantified the stress distribution in atypical proximal femurs implanted with a commercially available calcar guided short stem.
Methods
Geometrical shape variations in neck-shaft angle (NSA), neck-length (NL) and anteversion (AV), were determined three-dimensionally in the Mimics Innovation Suite (Materialise N.V., Leuven, Belgium) from a CT dataset of 96 segmented femurs. For each shape variation, the femurs that had the two lowest, two average and two highest values were included (18 femurs). Using scripting functionality in Mimics, CAD design files of the calcar guided Optimys short stem (Mathys, Bettlach, Switzerland) were automatically sized and aligned to restore the anatomical hip rotation center. Stem size and position were manually corrected by an orthopedic surgeon before finite element (FE) models were constructed using a non-manifold assembly approach (Figure 1). Material properties were estimated from the CT dataset and loads representing walking and stair climbing were applied [1]. Stress-shielding was evaluated by the change in average strain energy density pre- and post-operatively in three different regions (calcar, midstem, tip) each being subdivided in four quarters (medial, lateral, anterior, posterior) (Figure 2).
Results
Stress shielding in the proximal femur was seen in all models, especially in the calcar-medial region. In that region, the largest variation in stress shielding was observed for the models with an atypical NSA, ranging from 57% to 96%. The lowest amount was found in a patient with an average NSA (124°), and the highest amount was found in a patient with a small NSA (109°) (Figure 2). In the models selected for their varying neck lengths, calcar-medial stress shielding increased from 69% (NL 53 mm) to 97% (NL 66 mm). Stress shielding was least sensitive to variations in AV, ranging from 79% to 92%. Similar patterns were observed for walking and stair climbing loads.
Discussion
Stress shielding was smallest in femurs where the load-transfer between implant and bone was located more proximally, while higher levels of stress shielding occurred when the load transfer was more pronounced at the tip of the stem (Figure 3). Two femurs with an average NSA and NL showed substantially lower stress shielding than the 16 other femurs. This may suggest that the calcar guided Optimys short stem prevents stress shielding especially in average femurs, but less so in atypical femurs. Hence, a larger study population should be investigated to support this hypothesis.
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